This invention relates to electrical machines such as motors and hydrogenerators which have a stator core and a stator frame with a heat expansion difference between the core and frame, and more particularly, to a bulb generator which has a stator frame in direct contact with surrounding cold water.
During operation, the laminated stator core of electrical machines is always warmer than the supporting stator frame. This heat causes the core to expand radially more than the stator frame. The expansion difference causes high radial shrink forces which compress the core. The radial shrink forces lead to high tangential stresses. When these stresses exceed the stability limit of the core lamination, the core experiences buckling, which distorts the core lamination. FIG. 1 graphically illustrates the connection between the relative tangential deformation X of the core due to shrink forces and the tangential compression stresses S in the core lamination. Starting from zero tangential shortening--equivalent to zero temperature difference between the core and frame--the tangential stress increases rapidly to the stability limit SL. A further increase of temperature difference causes the lamination to break out perpendicular to the lamination which is called "buckling". Such buckling may happen in one or more places around the circumference of the core. The buckling manifests itself as a distortion in the lamination in the magnitude of about 0.25 to 1.0 inch and with a length of about 5 to 20 inches. The position of the buckling in the circumference is usually arbitrary.
Other parts of the circumference seem to be completely healthy. A further increase of temperature and tangential shortening to XM causes only a slight rise of the tangential stress because the buckling distortion accumulates in the tangentially expanded lamination. Reduction of the temperature will reduce the tangential stress to zero. However, the buckling deformation containing a certain amount of plastic deformation will not disappear. The residual deformation XR remains as permanant damage of the stator core lamination. It will be appreciated that it would be highly desirable to provide a lamination which is not susceptible to buckling.
In one arrangement designed to prevent buckling, the stator core lamination is supported by dovetail bars with spring connection to the frame. The expanding stator core lamination compresses the spring which reduces the shrink forces and the tangential stresses. A disadvantage of this configuration is that it is quite expensive because of the springs. Sometimes several hundred springs are needed to provide an even core support. In addition the springs, together with the core, act as a complex vibration system excited by the magnetic forces.
In another arrangement, the stator core has a certain gap relative to the frame. The development of the shrink forces are postponed because the heat expansion difference has to fill the radial gap prior to the build-up of the radial forces. The disadvantage of this approach is a lack of control with regard to the radial bending of the core relative to the frame. Vibrations and uneven magnetic pull between the rotor and parts of the stator core could occur.
In another approach, layers of the stator core lamination are glued to each other. This method could be used on both axial ends of the core or over a few inches of axial length or over the whole axial length. The many layers of glue will reduce the magnetic axial length of the core and reduce the efficiency. The glueing of each lamination is quite expensive both in material and labor. In addition, this method is limited to small and medium-sized machines. Accordingly, it would be appreciated that it would be highly desirable to provide a simple, inexpensive structure to prevent buckling of the core lamination.
Accordingly, it is an object of the present invention to provide a stator core structure which resists buckling.
Another object of the present invention is to provide a core lamination which resists buckling.
Another object of the invention is to provide a laminated core structure which resists buckling and is simple and inexpensive.